1. Field of the Invention
The present invention relates to a nonaqueous electrolyte rechargeable battery including a positive electrode, a negative electrode and a nonaqueous electrolyte, and more particularly to a nonaqueous electrolyte rechargeable battery using molybdenum oxide for positive electrode material.
2. Description of Related Art
Currently available rechargeable lithium batteries use lithium cobaltate (LiCoO2) or lithium manganate (LiMn2O4) for the positive electrode material and carbon materials for the negative electrode material. However, applications such as portable equipment demand rechargeable batteries capable of longer operation and thus having increased capacities and energy densities. Also, there has been a need in the art for alternative materials to lithium cobaltate which is a rare and expensive resource while being the most popular positive electrode material currently used.
Molybdenum oxide is considered to be a possible alternative to lithium cobaltate. In lithium cobaltate, an oxidation number of cobalt changes from trivalent to tetra-valent. An oxidation number of molybdenum, on the other hand, is changeable between tetravalent and hexavalent in molybdenum oxide. Accordingly, the use of molybdenum oxide in place of lithium cobaltate is expected to increase both capacities and energy densities of rechargeable batteries.
However, in the current state of the art, rechargeable lithium batteries using molybdenum oxide in place of lithium cobaltate only present the discharge capacity lower than the theoretical capacity. Japanese Patent Laying-Open Nos. Hei 11-250907 and Hei 3-88269 propose the use of molybdenum oxide in the amorphous form. However, resulting capacity and energy density have been still insufficient.
Also, it is known that when Li ions are inserted into molybdenum oxide, such Li ions move into spaces between layers composed of Mo and O and further into interiors of those layers to destruct them (See, for example, T. Tsumura and M. Inagaki, Solid State Ionics, vol.104 (1997), pp 183-189). This is considered due to the weak bond of Mo and O and has caused a problem of capacity decline with cycling for nonaqueous electrolyte rechargeable batteries using conventional molybdenum oxide for the positive electrode material. Since such a declining capacity with cycling is attributed basically to the weak bond between Mo and O the use of molybdenum oxide having an amorphous or other non-laminar crystal structure has also resulted in the capacity reduction with cycling.
It is an object of the present invention to provide a nonaqueous electrolyte rechargeable battery which uses molybdenum metal oxide for its positive electrode material, which has improved capacity and energy density and which exhibits excellent cycle performance characteristics.
The present invention provides a nonaqueous electrolyte rechargeable battery having a positive electrode, a negative electrode and a nonaqueous electrolyte. Characteristically, the positive electrode comprises molybdenum metal oxide deposited, in the form of a thin film, on an aluminum-containing substrate and represented by the formula Mo1xe2x88x92xMxO2+y (where M is at least one element selected from the group consisting of Ni, Co, Mn, Fe, Cu, Al, Mg, W, Sc, Ti, Zn, Ga, Ge, Nb, Rh, Pd and Sn, x satisfies the relationship 0.005xe2x89xa6xxe2x89xa60.5, and y satisfies the relationship 0.6xe2x89xa6yxe2x89xa61.2).
The molybdenum metal oxide used, in the form of a thin film, for the positive electrode in accordance with the present invention is represented by the formula Mo1xe2x88x92xMxO2+y, i.e., the molybdenum metal oxide derived via partial substitution of a metal element for Mo in molybdenum oxide. The substituting metal element M is at least one element selected from the group consisting of Ni, Co, Mn, Fe, Cu, Al, Mg, W, Sc, Ti, Zn, Ga, Ge, Nb, Rh, Pd and Sn. The partial substitution of metal element M for Mo increases a bond strength between the metal element and oxygen to thereby improve cycle characteristics. Preferably, the substituting element M is at least one element selected from the group consisting of Ni, Co, Mn, Fe, Al, Mg, W and Ti.
In the above-specified formula, x is a stoichiometric value of the substituting element M and satisfies the relationship 0.005xe2x89xa6xxe2x89xa60.5, preferably 0.01xe2x89xa6xxe2x89xa60.3. If the value of x falls outside the specified range, the sufficient improvement in cycle characteristics, which is an effect of the present invention, may not be obtained.
In the above-specified formula, y indicates a variation in stoichiometry of oxygen and satisfies the relationship 0.6xe2x89xa6yxe2x89xa61.2. If y is maintained at a value within this range, nonaqueous electrolyte rechargeable batteries result having improved capacities and energy densities.
A substrate surface over which the molybdenum metal oxide thin film is to be deposited contains aluminum and specifically comprises an aluminum metal or aluminum alloy. Preferably, the thin film may be deposited on the substrate by using a thin film-forming technique such as a CVD, sputtering, vacuum deposition, spraying process or the like.
In the present invention, the substrate preferably serves as a current collector for an electrode. Also, the substrate surface over which the molybdenum metal oxide thin film is to be deposited preferably has a surface roughness Ra in the range of 0.001-1 xcexcm. If the substrate having such a surface roughness Ra is used, the substrate serving as a current collector can maintain good adhesion to the molybdenum metal oxide thin film even during its expansion or shrinkage on charge-discharge cycling and thus collect current efficiently. The surface roughness Ra is defined by Japan Industrial Standards (JIS B 0601-1994) and can be determined as by a surface roughness meter.
In the present invention, the surface roughness Ra of the substrate preferably satisfies the relationship Raxe2x89xa6t, where t is a thickness of the molybdenum metal oxide thin film.
Also in the present invention, the surface roughness Ra of the substrate preferably satisfies the relationship Sxe2x89xa6100Ra, where S is an average interval of peaks in surface irregularities. The average peak interval S is also defined by Japan Industrial Standards (JIS B 0601-1994) and can also be determined as by a surface roughness meter.
In the present invention, the surface roughness Ra of the substrate is more preferably in the range of 0.0105 xcexcm and greater, still more preferably in the range of 0.011-0.1 xcexcm, still more preferably in the range of 0.012-0.09 xcexcm. The use of the substrate having a surface roughness Ra within the above-specified range enables structural control of the molybdenum metal oxide thin film deposited thereon, resulting in the formation of an electrode which exhibits improved cycle characteristics. That is, by depositing the molybdenum metal oxide thin film on the substrate roughened at its surface to the specified surface roughness Ra, the structure of the molybdenum metal oxide thin film can be rendered into such a form that enhances its adhesion to the substrate as a current collector.